1. Field of the Invention
The present invention relates to ophthalmic devices and techniques and more particularly to an orthokeratology system for correcting presbyopic errors. An optical parametric oscillation converter (OPO) produces tunable-wavelength photonic energy that penetrates the cornea to create "shallow-plane" intra-lamellar microweld effects or intercalation of lamellae while a "prosthetic" lens maintains the corneal surface in a optimal condition and prolate curvature. The prosthetic lens is further adapted to mediate cryo- and photonic waves that propagate through the cornea to create the desired shallow-plane microweld effects--the novel technique and system described as a thermal-adjunct orthokeratology system (TAOS).
2. Description of the Related Art
The prior art methods of correcting refractive errors can be classified into three general categories: (1) external/internal appliances such as spectacles, contact lenses and intraocular lenses (IOL); (2) the practice of orthokeratology which uses progressively different-shaped contact lenses as prostheses to mold corneal curvature--followed by use of a retainer contact lens; (3) refractive surgeries of several types, which include various laser-based such as PRK LASIK and LTK; and keratoplasty procedures. The present inventive TAOS technique (thermal-adjunct orthokeratology system) disclosed herein and is a combination of orthokeratology and keratoplasty and utilizes two components: (i) a novel corneal prosthetic lens to mold and protect the anterior corneal condition and shape, and (ii) energy delivery systems for creating novel "shallow-plane" lamellar weld-effects that differ from other laser-based modalities (PRK, LASIK, LTK) both in "effect-depth" in the cornea and in the "effect-characteristics" of the photonics-cornea interaction. Further, the TAOS system and technique is unique in that it is being developed specifically to treat presbyopia. Prior art laser refractive surgical methods (e.g., LASIK and PRK;) are well-suited for treating myopia, but thus far are not proven for treating hyperopia, and are not used for treating presbyopia. Therefore, the TAOS presbyopia procedure is adapted to be a "micro-invasive" technique such that it can be used as a maintenance therapy to be performed repeatedly as the patient's presbyopic condition intensifies, and the treatment effects moderate.
As background, refractive errors result from the inability of the eye's optic system, consisting of the dome-shaped cornea C and the crystalline lens LE just behind it to properly focus images on the retina, the nerve layer at the back of the eye. Approximately 80 percent of the refracting power of a human eye is within the cornea; about 20 percent is within the lens LE (see FIG. 1A). Refractive errors generally include myopia, hyperopia, presbyopia and astigmatisms. Myopia is a refractive error that causes poor distance vision, and is characterized by an elongate eye or steepened corneal shape wherein images focused in front of the retina. Hyperopia is the opposite, and is caused by a shortened eye or flattened cornea that focuses images beyond the retina.
Presbyopia, the focus of this disclosure, results from the aging process and is caused by a diminished ability of the lens LE to elastically change its shape, generally which begins after about age 35 to 40.
The TAOS presbyopia and hyperopia treatment techniques disclosed herein deal with increasing the optical power of the cornea C to compensate for a decrease in the maximum power of lens LE. In other words, to overcome a lack of power in lens LE, the cornea's power must be increased by making its curvature more prolate (steeper in paracentral zone PZ) since it is not possible to precisely alter corneal radii. (see FIGS. 1B). As further background, four variables determine the refractive power of the cornea: (i) the optical power of the cornea in diopters; (ii) the optical power of the lens; (iii) the depth of the anterior chamber between the cornea and lens, and (iv) the axial length of the globe, i.e., distance of cornea and lens to retina. Each of the above factors may fall within a statistical "normal range" within standard deviations along a bell-curve, and the "correlation" among the four components can result in a minimal refractive error of 0.25 diopter or less (emmetropia). However, the four factors may still each be in a "normal range" but have an incorrect "correlation" and result in a refractive error (correlation ametropia), for example, needing correction of from 1.0 to 4.0 diopters. It is believed that about 90% of refractive errors relate to correlation ametropias of 4.0 diopters or less. The TAOS technique is directed specifically toward this population of refractive errors of 1.0 to 4.0 diopters.
To understand the techniques of the present invention in increasing the power of the cornea (steepening the cornea) in a maintenance therapy--and to understand the shortcomings of the prior art--it is necessary to describe (i) the distinct role played by the Bowman's layer BL and anterior stromal lamellae ASL in corneal morphology; (ii) the role of the tear film TF in photonic-cornea interactions.
As can be seen in the not-to-scale corneal section of FIG. 2, the cornea includes several distinct layers with indicated thicknesses. The anterior surface of the cornea consists of the epithelial layer EP and cornea C is not at all smooth and is made up of myriad projections and ridges called microvilli MV and microplicae MP. In order to allow the cornea to function and refract light, the corneal tear film TF which is repeatedly spread over the epithelium EP by the eyelids to provide the necessary smooth refracting surface. As shown in FIG. 2, the tear film This about 10-12 .mu.m thick with an outermost lipid layer LIP that contacts the air and retards evaporation of the tear film. Whether a 12 .mu.m tear film TF is intact, partly intact, entirely evaporated or purposefully removed can markedly change the depth of photonic energy absorption within the cornea.
The next layers below the tear film TF/epithelium EP are the Bowman's layer BL and the stroma S. The Bowman's layer BL is an acellular layer of randomly-oriented and overlapping collagen fibrils CF. (see FIG. 2). The stroma S constitutes about 85-90% of the corneal thickness. The entire stroma S is comprised of about two hundred lamellae L which lie in flat sheets and extend from limbus to limbus. Between the lamellae are keratocytes layers KL, the constitutive cells of the cornea which produce the intra-fibril ground substance GS and support synthesis of collagen fibrils CF (see FIG. 3).
The Bowman's layer is from 15 .mu.m to 20 .mu.m thick and blends posteriorly into the stroma S, or more particularly into the anterior stromal lamellae ASL (see FIG. 3). For the purposes of this disclosure, the stromal lamellae are divided into three regions: the anterior stromal lamellae ASL; medial stromal lamellae MSL; and the posterior stromal lamellae PSL. In general, each lamella (sheet) consists of strong parallel collagen fibrils CF maintained in a spaced separation by ground substance GS. In the posterior stromal lamellae PSL, the collagen fibrils CF run at approximately "right angles" from one lamellae L to the next, and such posterior lamellae are distinct and non-interlocking. In contrast to the posterior lamellae PSL, the anterior stomal lamellae ASL have collagen fibrils CF are oriented at oblique angles, lamellae-to-lamellae. Further, the collagen fibrils CF of the anterior lamellae ASL interleave between over- and underlying lamellae. In other words, the most anterior lamellae ASL are interlocked to a significant extent and thus act similar to the Bowman's layer BL in which collagen fibrils are random and overlap. Such interleaving of fibrils in the anterior lamellae ASL can easily be seen in the anterior 1/4 to 1/3 of the stroma S (the ASL herein) as the layer appears gray-like in slit lamp microscopy.
It is believed that the Bowman's layer BL and the anterior stromal lamellae ASL, due to the interweaving of the collagen fibrils CF and the oblique cross-orientation of the fibrils, are the most important elements in maintaining the anterior corneal curvature in the "normal range" (.+-.7.7 mm radius). FIG. 3 shows the intercalation of the Bowman's layer BL and the anterior lamellae ASL, where the random collagen fibrils project into and entwine with the more layered ordering of stromal collagen fibrils. The Bowman's layer BL and anterior stromal lamellae ASL are unyielding to stretching forces and thus are adapted to contain the substantial intraocular pressure (IOP) of the eye. The TAOS technique disclosed herein of creating shallow-layer weld-effects focuses on these layers to create intercalations thereof, and therefore from time-to-time the layers will be referred to herein as the Bowman's+ASL region of the stroma S. Such unyielding characteristics of the Bowman's+ASL is evident in all cases of corneal swelling (e.g., stromal edema or prior art photonic interactions in the mid-stroma and posterior stroma) which causes the cornea to protrude severely posteriorly since the above-described lamellar interleaving or interdigitation is lacking poteriorly.
The prior art LTK method differs markedly from the TAOS technique disclosed herein when comparing the "effect-characteristics" at depths in the cornea. The stated objective of the prior art LTK method is to create so-called "shrinkage" of collagen fibrils into clumps in a series of spots around the cornea to create lines of tension between the spots to affect corneal curvature. The so-called "shrinkage" of corneal collagen fibrils occurs at from about 55.degree.-60.degree. C. (see Stringer, H., Parr, J., "Shrinkage Temperature of Eye Collagen", Nature 1964; 204:1307.; see also U.S. Pat. No. 4,976,709). The TAOS technique disclosed herein utilizes higher temperatures to cause the desired microweld-effects which cause fusions of, or lesions within, a lamellar region as described below.
What is needed is a non-contact photonics system for re-shaping corneal curvature (i) that provides a minimally invasive therapy, (ii) that causes minimal thermal insults to cornea, both in duration of exposure and the volume of tissue affected; (iii) that treats the most widespread refractive disorders, the first being presbyopia and the second being hyperopia, (iv) that provides a solution that is not dependent on transient lines-of-stress within the stroma (v) that reduces regression of effects; (v) that protects the epithelial layers from damage, (vi) that provides energy delivery systems that are highly individualizable for irregular or astigmatic corneas to reach the greatest number of candidates, and (vii) that provides a repeatable maintenance therapy to accommodate natural presbyopic age-effects of the patient's lens over his or her lifetime.